1. Field of the Invention
The present invention relates to a heterocyclic compound and a light-emitting element, a light-emitting device, a lighting device, and an electronic device using the heterocyclic compound.
2. Description of the Related Art
In recent years, research and development have been actively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a substance having a light-emitting property is interposed between a pair of electrodes. By application of a voltage to this element, light emission can be obtained from the substance having a light-emitting property.
Since such a light-emitting element is of self-light-emitting type, the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not needed, and so on. Therefore, such a light-emitting element is thought to be suitable as a flat panel display element. Besides, such a light-emitting element has advantages in that it can be formed to be thin and lightweight, and has quite fast response speed.
Furthermore, since such a light-emitting element can be formed in a film form, planar light emission can be easily obtained by formation of an element having a large area. This is a feature which is difficult to be obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Therefore, the light-emitting element has a high utility value as a surface light source applicable to illumination and the like.
Light-emitting elements using electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. When an organic compound is used as a light-emitting substance, by application of a voltage to a light-emitting element, electrons and holes are injected into a layer containing the light-emitting organic compound from a pair of electrodes, and thus a current flows. The carriers (electrons and holes) are recombined, and thus, the light-emitting organic compound is excited. When the light-emitting organic compound returns to a ground state from the excited state, light is emitted.
Because of such a mechanism, the light-emitting element is referred to as a current-excitation light-emitting element. Note that an excited state of an organic compound can be of two types: a singlet excited state and a triplet excited state, and light emission from the singlet excited state (S*) is referred to as fluorescence, and light emission from the triplet excited state (T*) is referred to as phosphorescence. In addition, the statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3.
At a room temperature, a compound capable of converting a singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound) exhibits only luminescence from the singlet excited state (fluorescence), not luminescence from the triplet excited state (phosphorescence). Accordingly, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
On the other hand, in a case of a light-emitting element including the phosphorescent compound described above, the internal quantum efficiency thereof can be improved to 75% to 100% in theory; namely, the emission efficiency thereof can be 3 to 4 times as much as that of the light-emitting element including a fluorescent compound. Therefore, the light-emitting element including a phosphorescent compound has been actively developed in recent years in order to achieve a highly-efficient light-emitting element (refer to Non-Patent Document 1).
When a light-emitting layer of a light-emitting element is formed using the above phosphorescent compound, for suppression of the concentration quenching of the phosphorescent compound and the quenching due to triplet-triplet annihilation, the light-emitting layer is formed so that the phosphorescent compound is dispersed throughout a matrix formed of another material in many cases. In this case, the material used for forming the matrix is called a host material, and the material dispersed throughout the matrix like the phosphorescent material is called a guest material.
When a phosphorescent compound is used for a guest material, a host material is required to have higher triplet excitation energy (a difference in energy between the ground state and the triplet excited state) than the phosphorescent compound. CBP, which is used as the host material in Non-Patent Document 1, is known to have higher triplet excitation energy than a phosphorescent compound which exhibits light emission of green to red and is widely used as a host material with the phosphorescent compound.
However, although CBP has high triplet excitation energy, it is poor in capability to receive holes or electrons; therefore, there is a problem in that driving voltage is increased. Therefore, a substance which has high triplet excitation energy and also can easily accept or transport both holes and electrons (i.e., a bipolar substance) is needed as a host material for a phosphorescent compound.
Furthermore, since the singlet excitation energy (the difference in energy between the ground state and the singlet excited state) is higher than the triplet excitation energy, a substance which has high triplet excitation energy also has high singlet excitation energy. Therefore, a substance which has high triplet excitation energy and a bipolar property as described above is also effective as a host material in a light-emitting element using a fluorescent compound as a light-emitting substance.